1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly, to an improved transflective liquid crystal display viewable under all lighting conditions, such as total dark, indoor lighting, in shade, medium sunlight, strong sunlight, and direct sunlight, without excessive power consumption.
2. Description of the Related Art
Many features of liquid crystal displays (LCDs), such as light weight and size, low power consumption and high resolution, make LCDs a popular choice in various electronic applications. These applications include digital cameras, palm PCs, notebook computers, tablet PCs, workstations, and navigation systems in automobiles, marine vessels, and airplanes. Most of these applications are portable and can be transited between indoor and outdoor. Thus, there is a need to develop a display to accommodate both indoor and outdoor environments and perform regardless of different lighting conditions. Various types of LCDs have evolved around this need.
With reference to FIG. 1, a conventional transmissive liquid crystal display (LCD) is shown. The LCD includes a liquid crystal cell 100 comprising a front transparent electrode with color filters 101, a rear transparent electrode (pixel portions) 102, and a layer of liquid crystals 103 between the front and rear transparent electrodes. The liquid crystal cell 100 is usually sandwiched by a front glass substrate 104 and a rear glass substrate 105. A first dichroic polarizer 106 adheres to the front surface of the front glass 104. Likewise, a rear dichroic polarizer 107 adheres to the rear surface of the rear glass 105. The transmissive display further includes a backlight cell assembly 108. A regular LCD contains 1 to 4 lamps that provide between 100 and 300 nits of illumination 110 at the surface of LCD. This level of brightness enables this type of LCD to perform beautifully indoors. In an outdoor setting, the anti-glare surface of the first polarizer 106 reflects and diffuses about 3% to 5% of the ambient sunlight A to a viewer's eyes. The amount of background reflection 109 is strong, overwhelming the illumination 110 from the backlight 108 and obscuring the image generated by the LCD.
One approach used to improve the performance of this type of LCD under sunlight is to apply an anti-reflection coating on the front surface. Although providing some improvement, the anti-reflection coating alone is not sufficient to provide an LCD viewable under direct sunlight. Further improvement is necessary.
Another solution commonly adopted is to increase the illumination of transmissive LCDs for outdoor application by adding more lamps to the backlight cell. The term “high-bright LCD” describes this modified transmissive LCD. In general, an LCD requires at least 1000 nits of illumination to be viewable under sunlight. To reach this level of brightness, an LCD requires 10 to 12 lamps. The additional lamps consume more power, generate excessive heat, experience contrast washout and require dimension and circuit alterations. Alterations of the LCD's dimensions and circuits are costly. Thus, high bright LCDs generally create more problems than they solve.
Referring now to FIG. 2, a common construction of a reflective LCD is shown. A reflective LCD does not have problems with power consumption since ambient light A is used for illumination. A reflector 201 is positioned behind a liquid crystal display assembly 204. Generally, the reflector 201 is an opaque surface of highly reflective material (such as aluminum or silver) with 90% to 98% reflection. The LCD display assembly 204 may also contain a second dichroic polarizer (not shown). A portion of ambient light 202 passes the liquid crystal display assembly and reaches the reflective surface of reflector 201. The reflector 201 reflects ambient light portion 202 and uses it as the display's illumination 203. Because the display's illumination is tied to the amount of ambient light provided, the visibility of reflective LCD is highly surrounding-sensitive. Under strong ambient light, the LCD has good illumination. However, LCD brightness diminishes as ambient light decreases. This disadvantage of the reflective LCD strongly limits its applications.
With reference to FIGS. 3A and 3B, a “transflective LCD” is shown. The transflective LCD was developed to overcome the shortcomings of the reflective LCD. A major element of the transflective LCD is the “transflector”, which is partially transmissive and partially reflective. The transflector uses ambient light and/or a backlight to illuminate the LCD. One type of transflective LCD implements the transflector as a series of electrodes 301, where the electrodes 301 are imbedded within the compartment of pixel portions 102 of the liquid crystal cell 100. FIG. 3A shows the structure of a transflective LCD with transflective electrodes 301. In FIG. 3B, the cropped partial area of the pixel portions 102 with transflective electrodes 301 is shown. The transflective electrodes 301 have highly reflective regions 301r and transmissive portions 301t contacting the transparent electrodes of pixel portions 102. When ambient light A is not strong, the transmissive portions 301t allow the transmission of light B from backlight cell 108 as the illumination 302 of LCD. When ambient light A is strong, the reflective portions 301r reflect ambient light 303 entering the liquid crystal panel 100, and send it back out as illumination 304 of LCD.
Still referring to FIGS. 3A and 3B, the visibility of the LCD is excellent when the ambient light A is strong. However, the combination of reflective portions 301r and transmissive portions 301t within the same domain (pixel portions 102) imposes undesirable features on the LCD. The problems are more noticeable when the LCD is used indoors, and include low brightness, loss of color, low contrast and a narrow viewing angle. In addition, pixel size of the LCD is limited by the need to accommodate both transmissive and reflective electrodes. The limited pixel size results in increased manufacturing difficulties and costs for higher resolutions.
Another type of transflective LCD comprises a transflective plastic film as the transflector, positioned in the rear of liquid crystal panel (not shown). Although easy to construct, this type of transflective LCD has inefficient illumination. The commonly used transflective films normally have 20% to 40% transmission efficiency and 50% to 70% reflection efficiency. Thus, this type of transflective LCD is not as bright as either purely reflective or purely transmissive LCD types.
In summary, a regular liquid crystal display can have satisfactory performance either indoors or outdoors. A high bright LCD, though acceptable for both indoor and outdoor applications, consumes high power and demands various complimentary re-designs of the device system to accommodate the excessive heat. Reflective LCDs do not perform well indoors. Transflective LCDs are limited by pixel size and do not perform optimally under certain ambient light. Thus, there is a great need to develop a liquid crystal display assembly that consumes low power without excessive heat generation, and has good color, adequate brightness and sufficient contrast under all lighting conditions.